Aug. 1, 2011 -- Researchers seeking to explain the rising number of asthma cases in children have fingered a new suspect: electromagnetic fields (EMFs), energy that can't been seen or felt that is generated by household appliances, electronic devices, cars, and power lines.

In a study, they found that babies born to women who are exposed to stronger EMFs during pregnancy had more than triple the risk of developing asthma compared to babies born to women exposed to weaker EMFs.

In other words, about 13% of children born to women in the group with the lowest EMF exposures developed asthma compared to about 33% of children born to women who had high EMF exposures.

"That's a striking figure," says David Savitz, PhD, a professor of community health and obstetrics and gynecology at Brown University in Providence, R.I. "That magnitude of association we don't see very often. If it was correct, and that's a big 'if,' that would be really startling."

Savitz, who has studied the health effects of electromagnetic fields but was not involved in the research, says that while the finding is interesting, there's no reason to give up using a hair dryer or microwave just yet.

He says that unlike contaminants like cigarette smoke or lead that are known to be dangerous, there's little evidence that low-frequency EMFs, the kind measured in the study, are harmful.

"This has been very, very thoroughly studied, and it really is questionable whether it causes any health effects at any reasonable level," Savitz tells WebMD. "It's certainly not something that falls into the category of a known hazard."

But Savitz and others acknowledge that all research has to start somewhere.

"There are a lot of important topics that started out looking pretty flaky and pretty unlikely. There was a time when it made no sense that smoking could be bad for you," he says.

Other experts agree.

"The study appears to be well executed and the finding is surprising," says Jonathan M. Samet, MD, a pulmonologist and epidemiologist at the Keck School of Medicine at the University of Southern California in Los Angeles.

Samet recently led a World Health Organization panel that concluded that EMFs from cell phones and other wireless devices could possibly cause cancer.

The current study didn't account for EMFs from cell phones or wireless networks, which emit higher-energy frequencies than were measured in the study.

Samet says that based on what we know about the development of asthma, it's hard to understand how EMFs might play a role. Repeating the study, he says, will be an important next step.

Previous studies have shown that EMFs may adversely affect the immune system.

Linking EMFs to Asthma in Kids

Researchers asked pregnant women who were members of the Kaiser Permanente Northern California health plan to wear magnetic field sensors around their waists for 24 hours.

The sensors took readings every 10 seconds, recording magnetic field levels of everything the women came into contact with during the day.

The sensors measured low frequency magnetic fields, which are generated by things like refrigerators, vacuum cleaners, hair dryers, cars, power lines, stoves, microwaves, computers, nearly anything that can be plugged in or runs on a motor.

They did not measure magnetic fields generated by cell phones or wireless networks, which operate at higher frequencies.

The sensors generated a total of 8,640 readings for each mother and baby.

Researchers then ranked those readings from the highest to lowest and picked out the middle number as a way to judge exposure.

Researchers don't know why some women had higher exposures while others had lower exposures, but Savitz says roughly 10% to 20% of households in the U.S. would meet the criteria for high EMF exposures used in the study.

Researchers then followed the women and their children for up to 13 years.

Children were considered to have asthma if a doctor diagnosed them with the condition twice in the same year.

Compared to children of mothers in the low magnetic field group, who developed asthma at rates that were roughly comparable to the national average, those in the high group had a 350% increased risk of getting the condition, while those in the medium group had a 74% increased risk.

The association remained even after researchers adjusted their data for things that might independently influence the development of asthma in kids, like age, sex, early birth, low birth weight, breastfeeding, and a family history of the condition.

Researchers say women who are worried about EMFs can do simple things to lower their exposure.

"The problem with EMF is that you can't see, smell it, you can't touch it," says study researcher De-Kun Li, MD, PhD, a reproductive and perinatal epidemiologist at the Kaiser Permanente Division of Research in Oakland, Calif. "But you can avoid those sources that we know about."

"The great thing about EMF is that distance really helps," Li says. For example, "When you turn the microwave on, don't stand right next to it. Try to, when you use a hair dryer, try to use it far away from your tummy as much as you can."

In the case of can openers, opting for a hand crank device, rather than an electric one, can lower EMF exposure.

In the case of vacuum cleaners, the study may be a good excuse to hand off the job to your partner.

The study is published in the Archives of Pediatric and Adolescent Medicine.